1. Field of the Invention
The present invention relates to a vehicle periphery monitoring apparatus that monitors the periphery of a vehicle based on an approaching object captured by a camera installed in the vehicle.
2. Description of the Related Art
An example of apparatuses that monitor the periphery of a vehicle based on images picked up by a camera installed in the vehicle is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-43494 (hereinafter referred to as Patent Document 1). In this vehicle peripheral monitoring apparatus, as shown in a conceptual diagram of FIG. 1, cameras (nose view cameras) incorporated in the front of a vehicle pick up side views FL and FR in front of the vehicle, which are likely to be blind spots as viewed from a driver seat. The picked-up side views are displayed on a monitor at the driver seat in the vehicle so that the driver can confirm safety, for example, when advancing the vehicle into an intersection.
Also, in recent years, apparatuses that detect a moving object from images picked up by a camera installed in a vehicle using an optical flow method have been proposed in, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2004-198211 (hereinafter referred to as Patent Document 2), Japanese Laid-Open Patent Publication (Kokai) No. 2004-56763 (hereinafter referred to as Patent Document 3), Japanese Laid-Open Patent Publication (Kokai) No. 2005-276056 (hereinafter referred to as Patent Document 4), and Japanese Laid-Open Patent Publication (Kokai) No. 2001-39248 (hereinafter referred to as Patent Document 5).
A movable body circumstance monitoring apparatus disclosed in Patent Document 2 is intended to identify the position and posture of a movable body without error accumulation and with high accuracy using a single image pickup means to ensure visibility at a desired position of the movable body. The apparatus extracts at least four feature points within a specific plane from an image of a peripheral area picked up by the single image pickup means during movement. When the movable body shifts from a first state to a second state, the apparatus causes a feature tracking means to track the extracted feature points, and causes a moving state identifying means to identify the relative position and posture of the image pickup means based on the plane coordinates of the feature points of the images picked up in the first and second states. Further, the apparatus causes a three-dimensional coordinate estimating means to estimate the three-dimensional coordinates of a feature point of a predetermined object based on the positions and postures of the image pickup means in the first and second states, the plane coordinates of the feature point of the predetermined object in the image picked up in the first state, and the plane coordinates of the feature point of the predetermined object in the image picked up in the second state.
A monitoring apparatus disclosed in Patent Document 3 is a vehicle monitoring apparatus that detects an approaching object using the optical flow method. The apparatus is intended to detect an approaching object with high accuracy even when a vehicle equipped with the monitoring apparatus is taking a curve. The apparatus calculates optical flows of feature points of a detected approaching object using a multi-resolution method or a pyramid method. In the multi-resolution method, a plurality of images with different resolutions are prepared in a hierarchical manner with respect to one image, optical flows are obtained by performing pattern matching in order from the roughest image to the finest image.
On the other hand, a nose view monitoring apparatus disclosed in Patent Document 4 calculates optical flows with respect to extracted feature points, and selects only feature points having a vector in an approaching direction as an approaching feature from among feature points of right and left views.
A vehicle camera apparatus disclosed in Patent Document 5 measures the distance to a moving object using a combination of a distance sensor and a camera.
In the above conventional monitoring apparatuses, however, an image picked up by the camera is basically a single view image. To find the distance to a moving object from a single view image, some sort of presumption or limitation is required. As an example of such presumption or limitation, there is a method in which a road plane within an image is presumed so as to find the distance to a moving object from a single view image. In this method, a ground plane of a moving object on an image is found, and the distance to a moving object is found from camera parameters (position, direction, focal length, etc.) and the position of the ground plane on the image. In general, however, it is difficult to recognize a ground plane on an image. Therefore, a ground plane to be recognized is limited to a ground plane of an automobile on an image shot from the front or rear. In general, however, it is difficult to view an object diagonally and recognizing a ground plane in order to recognize an approaching object included in the visual field as in the case of the above-mentioned nose view monitoring apparatus.
Also, it cannot be denied that measuring the distance to a moving object using a combination of a distance sensor and a camera as in the case of the vehicle camera apparatus disclosed in Patent Document 5 makes the apparatus expensive.